One of Britain's leading constitutional experts said yesterday that Scotland Yard had the right to raid the offices of Tory frontbencher Damian Green.

Vernon Bogdanor, professor of government at Oxford University where one of his undergraduates was David Cameron, dismissed claims from Labour backbenchers who said the Metropolitan Police had breached parliamentary privilege by raiding Green's office in the House of Commons.

Bogdanor said that parliamentary privilege extended only to what an MP said in the Commons and that the Yard had a responsibility to arrest individuals over allegations of illegality regardless of whether they were an MP or not.

'MPs are subject to criminal law as much as the rest of us,' he said. 'Their parliamentary privilege only extends to speeches in the chamber, not their offices.

'If an MP were accused of theft and keeping stolen goods in his office at the House of Commons, should he be exempt from a police investigation?'

Bogdanor guessed that Green may have been arrested because he had 'unwittingly' disclosed sensitive security information. 'If you and I were accused of that, then we also would be arrested', he added.

As pressure mounted on the Speaker, Michael Martin, for allowing police into the Palace of Westminster to search Green's Commons office, Bogdanor said: 'The Speaker may be asked to explain his reasons why he agreed for police to search the offices. If you have nothing to hide, then that is fine.'

Bogdanor added that the investigation into the Whitehall leak could not be interpreted as evidence that Britain had become a 'police state'.

'If it were a police state, then it would be like a communist state where MPs would be exempt from any laws, and here they are not,' he said.